This invention relates to amplifier circuitry and more particularly to amplifier circuitry including means for detecting and/or preventing the application of input signals that exceed the amplifier linear operating capability.
In many situations wherein an amplifier circuit is utilized to provide signal gain, it is either advantageous or necessary to detect and/or prevent non-linear operation of the amplifier, i.e. operation in which the amplifier input and output signals are not linearly related, and hence the output signal is not a faithful reproduction of the input signal. Basically, non-linear amplifier operation can result from two distinct signal conditions. First, if the magnitude of the input signal exceeds a certain value, the amplifier will not be capable of producing an output signal equal to the magnitude of the input signal multiplied by the amplifier gain. This condition can result either from applying an input signal that exceeds the amplifier design capability, or by improperly loading the amplifier with a load impedance not within the amplifier design capability. In such an instance it is generally said that the dynamic range of the amplifier has been exceeded and that the amplifier is overloaded. The second type of non-linear amplifier operation results if the amplifier cannot produce an output signal having a time rate of change substantially identical to the product of the time rate of change of the input signal multiplied by the amplifier gain. When an input signal having a time rate of change that exceeds the amplifier capability is applied to the amplifier circuit it is generally said that the amplifier slewing capability has been exceeded or that the amplifier is slew limited.
Non-linear operation of an amplifier can cause several generally deleterious effects. For example, severe harmonic distortion may occur, large undesirable signal transients may be generated, or the amplifier gain and bandwidth may be altered. In some instances, these conditions can cause the amplifier circuit to fail due to excess voltage stress within the amplifier circuitry or due to increased thermal dissipation demands. Further, when the amplifier is utilized as a portion of a larger electronic system, non-linear amplifier operation can cause a system malfunction by the generation of undesired output signal components or can cause failure of other system components by the generation of signals that exceed their component signal handling capabilities.
One approach to eliminating the non-linear amplifier operation problem has been the design of amplifier circuits having a dynamic range and slewing capability that exceeds the requirements of the input signals that will be applied to the amplifier input terminals. Although modern amplifier circuits -- especially that class of amplifier circuits generally known as operational amplifiers -- exhibit performance characteristics compatable with a wide range of input signals, many applications exist in which it is not possible to design an amplifier capable of handling every type of input signal that may be encountered. Further, even though an amplifier is designed to linearly amplify all signals that normally arrive at the input terminals, certain conditions can occur in which an input signal not within the linear range of the amplifier reaches the amplifier circuit. For example, in amplifiers utilized within electronic systems or electronic test instruments, even a skilled operator can inadvertently connect or operate the equipment such that the amplifier receives an improper input signal.
Prior art attempts to protect an amplifier when an improper input signal is applied have generally included clamping circuits to limit the signal voltage at one circuit point or another and have also included current-activated switches to disable the amplifier circuit or modify its operation when predetermined limits are exceeded. Although such attempts have been successful to some degree, several disadvantages have been encountered. First, those circuits that dynamically respond to non-linear operation often have not been fast enough to sense the nonlinear operation and apply corrective measures such as removing or reducing the input signal before a harmful output signal has been generated. Secondly, those circuits that rely on limiting the signal flowing through the amplifier often themselves exhibit non-linear transfer characteristics and, accordingly can be utilized only by adversely influencing the performance characteristics of the basic amplifier circuit. Thirdly, the prior art has not provided a non-linear operation system applicable for use with amplifier circuits comprising various circuit stages realized by silicon integrated circuits. This consideration is of prime importance since in many instances, the most expedious and economical amplifier design is accomplished with commercially available integrated circuits that can be used as "building blocks" to provide the desired input-output relationship. Nor has the prior art provided circuit means for detecting non-linear amplifier operation that is amenable to realization within an integrated circuit amplifier.
Accordingly it is an object of this invention to provide an amplifier circuit including means for detecting non-linear operation of the amplifier.
It is another and related object of this invention to provide an operational amplifier circuit that includes circuitry for producing an electrical signal indicative of whether the applied amplifier input signal exceeds the dynamic range and/or the slewing capability of the amplifier.
It is yet another object of this invention to provide an amplifier circuit that includes the detection and utilization of non-linear circuit operation such that the input signal is removed or reduced prior to the generation of substantial non-linear output signals.
It is still another object of this invention to provide an amplifier circuit including the detection of non-linear amplifier operation that is amenable to realization by the use of circuit stages including integrated circuits and is also amenable to realization as an integrated amplifier circuit including such non-linear detection means.